Project Summary Mast cells (MCs) are effector cells in asthma and their activation causes secretion of cysteinyl leukotrienes (cys-LTs) and prostaglandins (PGs). MCs not only secrete these mediators, but they also possess receptors for them, to perceive their signals. Cys-LTs are potent bronchoconstrictors, powerful inducers of vascular leakage, potentiators of airway hyper-responsiveness and play an important role in asthma and other inflammatory disorders. On the other hand, Prostaglandin E2 (PGE2) function in the field of asthma is controversial acting both as pro as well as anti-inflammatory, depending mainly on the receptor/s through which it exerts its effect. Our preliminary results indicate that cys-LTs together with PGE2 synergistically potentiate vascular inflammation through their action on MCs (enhanced calcium flux, c-Fos, COX-2, PGD2 and Macrophage Inflammatory Protein-1? (MIP-1?; CCL4)) generation. Interestingly, LTD4-PGE2 synergism is blocked only by combined treatment of CysLT1R antagonist (MK571/ singulair) and EP3 antagonist (L-798), suggesting the need for a combination of CysLT1R antagonists and EP3 blockers to treat inflammation in asthma. Further, PPAR? inhibitor, GW9662 inhibited this synergy along with PKG inhibitor, KT5823 and MEK inhibitor PD98059, implicating a role for PPAR?, PKG and Erk. Furthermore, LTD4+PGE2 synergism also potentiates pulmonary inflammation in der f sensitized mice (recruitment of immune cells, goblet cell metaplasia, up-regulation of inflammatory transcripts). Based on these preliminary data, we hypothesize that LTD4 has a potential to switch PGE2 signals from EP2/Gs/cAMP/PKA pathway to EP3/Gi/cGMP/PKG axis, generating pro-inflammatory signals and MC activation via PPAR?, COX-2 and PGD2, followed by potentiating pulmonary inflammation in der f- challenged mice. We will test this hypothesis in the following specific aims 1) To determine the mechanism by which Cys-LT/PGE2 synergism induces MC activation and PGD2 production in vitro with focus on PPAR? 2) To determine the physiological significance of cys-LT-PGE2 interactions in pulmonary inflammation in vivo in Der f-challenged mice, analyzing pathologic, physiologic, and immunologic signatures of the immune response and evaluate the contribution of MCs. These studies will carry substantial pathogenic and therapeutic implications for asthma and allergic diseases as well as provide the basis for development and translation of future therapeutic molecules that regulate inflammation.